This invention relates to signal acquisition and more particularly to acquisition of electrocardiograms.
Surface electrocardiograms are derived from electrical activity of nerves and muscles of the heart which extend to the surface of the skin. To obtain ECG signals, three or more electrodes are connected to selected locations on the patient's body. Pairs of patient electrodes are connected to differential amplifiers to provide one or more channels of ECG "leads" which are wave forms depicting electrical activity of the heart viewed along specific axes. An additional electrode is required to eliminate the common mode voltage which exists between the patient and the recording instrumentation and is common to all of the ECG signals.
Cardiac disorders that can be identified by electrocardiograms are either transient or persistent. Resting ECG's, which are performed in a doctor's office or a hospital, can detect persistent disorders which are identifiable in almost every heart beat if viewed at the correct angle. In order to determine the cause of transient disorders that occur rarely or cannot be reproduced in the doctor's office, such as ischemia and arrhythmia, ECG signals must be recorded for longer periods, typically 24 hours. In the event it is desired to obtain such signals while the patient performs his or her routine activities, the patient is connected to a portable ECG recorder commonly called an ambulatory or Holter monitor.
An ECG is a complex signal of irregular shape consisting of P, QRS and T complexes. The portion between the S and T complexes is called the ST segment. Deviations in the shape and rhythm of the ECG wave form from established norms are an indication of various cardiac abnormalities. For example, elevation or depression of the ST segment is an indication of ischemia, or an insufficiency in the supply of oxygen to the cardiac muscle.
ECG signals are often contaminated with electrical noise, some of which are external, such as power line interference. Other noise signals are caused by the patient. For example, such activity as the movement of skeletal muscles cause a high frequency noise signal. In addition, low frequency noise signals typically result from patient respiration or the slowly changing potentials caused by the electrode skin interface. These low frequency noise signals are often referred to as base line sway and generally occur at less than one Hz in the signal frequency spectrum.
If the ECG signal is to be recorded without distortion, the recording equipment must have an unvarying or flat amplitude response over the range of frequencies present in the signal. In addition, the system must have a linear phase response over the required frequency range so that all components passing through the recorder are delayed by the same amount. If the phase response is non-linear, components of the P, QRS and T complexes are delayed selectively and distortion will result.
In order to provide sufficient data for diagnosis, Holter monitors normally record several channels of ECG data, each of which is a view of the heart along a different axis. However, if base line sway, caused by low frequency noise signals, is permitted to occur, the various printed ECG signals become difficult to interpret if displacement of the signals in the different recording channels interfere one with the other. Therefore, Holter recorders generally include high pass filters to remove low frequency signals which cause base line sway. However, the use of such high pass filters to correct for base line drift tend to cause the ST segment of the ECG signal to be displaced in a direction opposite that of the QRS complex. This can cause the ST segment to be distorted so that an incorrect reading of the ST segment elevation may result.